The present invention relates to a method for manufacturing a diffraction grating formed by periodic corrugations, through utilization of two-beam interference exposure, and more particularly to a method of making a diffraction grating of a structure in which corrugations are reversed in phase between two adjacent regions.
Since a diffraction grating formed by periodic corrugations reflects therefrom or passes therethrough light of a desired wavelength alone, it has found use, in the field of optical communications, as a filter or an internal element of distributed feedback semiconductor lasers (which will hereinafter referred to simply as "DFB" lasers).
Of such lasers, a DFB laser of the type having a diffraction grating disposed in or near its light emitting region emits light of a single longitudinal mode; hence, this laser has been highlighted as a light source for optical communications, and a variety of proposals have been made thereon. Especially in recent years, it has attracted attention to reverse the phase of corrugations in the vicinity of the central portion of the diffraction grating for further stabilization of the single mode operation.
The oscillation wavelength of such a DFB laser is determined by the period .LAMBDA. of the corrugations of the diffraction grating, and the stability of its operation depends upon the accuracy of fabrication of the diffraction grating. Accordingly, the accuracy of fabrication of the diffraction grating will influence the characteristics of the DFB laser.